Short answers, pulled from the story.
The city of Durham was founded in 995 CE by monks carrying the relics of St Cuthbert. They chose the site at Dunholm because the River Wear curves around it in a horseshoe shape, making it defensively favourable.
The County Palatine of Durham was a semi-autonomous territory governed by the Bishops of Durham, who from 1075 held the right to raise an army, mint coins, levy taxes, and appoint their own judges and sheriff. The palatine jurisdiction was formally transferred to the Crown on the 5th of July 1836 under the Durham (County Palatine) Act.
The Stockton and Darlington Railway, which opened in 1825, was the world's first public railway to use steam locomotives. It ran through County Durham and marked the beginning of the modern railway era.
Miners came from across the UK and Ireland. Tens of thousands arrived from Cornwall between 1815 and the First World War, drawn partly by their experience of tin mining. Significant numbers also came from Northumberland, Cumberland, South Wales, Scotland and Ireland.
The Spennymoor Settlement, also called the Pitman's Academy, opened in 1930 and provided creative and educational opportunities to unemployed miners during the Great Depression. Notable former members include artists Norman Cornish and Tom McGuinness, writer Sid Chaplin, and journalist Arnold Hadwin.
County Durham registered its unofficial flag with the Flag Institute on the 21st of November 2013. The flag features St Cuthbert's cross counterchanged with the county's blue and gold colours, and was designed by Katie, Holly and James Moffatt.